1. Field of Invention
This invention relates to supplying power to an alternating current (AC) load, particularly an AC grid, from a direct current (DC) source. More particularly, the invention relates to methods and apparatus for controlling switching legs of a switching circuit of an inverter to supply high power factor electrical power to a load.
2. Description of Related Art
As energy demands of consumers and industry increase, alternative forms of energy are becoming increasingly accepted and developed. One of these alternative forms of energy is solar energy. Solar energy is often harvested through the use of one or more photovoltaic panels, which produce direct current (DC) power from the sun's energy. The sun's energy impinges upon a photovoltaic panel and can provide a readily available energy source in climates subject to high amounts of solar radiation. A photovoltaic panel or a plurality of such panels, known as an array, can produce more energy than may be required or demanded therefrom and thus while energy is available, it may not be used. To avoid wasting or not making use of this readily available energy, provisions are often made to supply electrical power into the AC utility grid from which most consumers and industry draw power. In some areas, photovoltaic arrays are totally devoted to supplying power to the AC utility grid.
Standards have been developed for supplying power to the AC grid, to ensure power is supplied in a manner that will not disrupt the operation of the grid or devices connected thereto. In particular, standards have been set for the phase relationship of voltage and current produced by devices that supply power to the grid. These standards require power to be supplied at near unity power factor, i.e. voltage and current in phase with that of the grid. Consequently, devices that supply power to the grid often have AC power factor correction circuits to ensure the output voltage/output current phase relationship is at or near unity. Typically, these circuits are implemented in digital or analog designs. Digital designs often employ digital signal processors or microprocessors. However, such implementations usually lack a cycle-by-cycle overcurrent protection loop, enabling an overcurrent condition to exist for an unacceptably long period of time, which can damage equipment. Analog or hybrid analog/digital designs typically require the use of two separate analog multipliers associated with respective phases of the AC waveform, each of which introduces errors into their product by different amounts, which tends to introduce a DC offset current to the output AC waveform, requiring additional compensation circuitry which can increase the complexity and cost of supplying power to the grid.